Optical industry and optical manufacturers commonly use the outer diameter (OD) of lenses to align them with an accuracy of a few μm (3-10 μm). Centring a stack of lenses is performed via an accurate mechanical datum that is built in the lens barrel. This is a very cost effective solution. The method is applied, for example, for aligning lenses of an imaging system. The imaging system may be, for example, part of a mobile phone. The approach of passive lens alignment implies that:
all lenses have been manufactured in order to align within a few μm (2-5 μm) the optical axis with respect to the OD of the lenses. This can involve some constraints on the design of the system itself or can require adding an extra step in the manufacturing process through a centring operation (ex: machining).the edges of the lenses are thick enough to form a good reference, for example >˜0.3 mm thick; andthe OD is accessible to provide an accurate mechanical datum.
FIGS. 1A and 1B show some examples of passive lens alignment using the outer diameter of lenses 10, aligned within a lens barrel 12.
Sometimes, opto-mechanical designers prefer to get the optical axis reference straight through the optical surface. FIG. 2 illustrates an example of passive lens alignment using the optical axis 14 of the lenses 10 as alignment reference. Still, this is not always possible, for example due to surface curvatures of lenses that might be not adapted to perform accurate centring. Also, the lens stack obtained from that kind of passive alignment can present undesirable stray light (flare, ghosts).
In some cases, passive alignment is not possible and active alignment is needed that definitely adds a long step to the process and brings the final cost up. In particular, this is the case when using a liquid lens with a curved window to cover the liquid chamber. Such liquid lens is described, for example, in European Patent Application No EP 1662276 in the name of the Applicant. The different constraints lead to an optical-mechanical design that requires active alignment of the liquid lens with respect to the rest of the lens stack.
Moreover, in a mobile phone camera, the space to integrate a camera module is very limited and there is no easy way to integrate a liquid lens in a lens barrel of such camera module. This is due to the fact that the liquid lens—whose actual packaging is about 7.75 mm OD—needs to be arranged on the object side to get the biggest focusing efficiency (range and speed). Such lens can not be integrated in a regular front aperture lens barrel concept where the front lens is always much smaller than the last lens (closest one to the sensor) whose OD is slightly smaller than the diagonal of the sensor (typically a 1/2.5 inch sensor.
FIG. 3 shows a liquid lens assembly 100 that may be implemented with a 5M (5 mega pixels) ⅓ inch sensor (or a bigger sensor like 1/2.5 inch) lens module for mobile application. The liquid lens assembly 100 includes a front lens 110 and a transparent window 112, which are facing each other and delimiting, in part, an internal volume containing two immiscible liquids 114, 116 with different optical indices. The front lens 110 serves as a front window of the liquid lens. One of the liquids is preferably an insulating liquid, for example comprising oil and/or an oily substance, and the other is preferably a conductive liquid comprising, for example, an aqueous solution. The front lens 110 and the window 112 are preferably formed of an optical transparent material such as glass. The liquid lens assembly 100 further comprises a cap 120 and a body 122 also acting as the electrodes of the liquid lens. The liquid lens assembly further includes a lens barrel 101.
Because the optical axis of the front lens is not easily and efficiently available due the structure of the liquid lens package, an active alignment of the liquid lens and the front lens is usually required to insure good optical quality. In active alignment, a lens is moved and centred with respect to the optical axis of the rest of the lens stack. For example, to actively align the liquid lens and the front lens with the required centring accuracy, a Modulation Transfer Function (MTF) measurement device can be used. Via a feedback loop between the measurement device and a micrometric actuator, the MTF measurements can be used to balance and optimize the MTF in the field of view.
A liquid lens and centring thereof in a lens barrel is described in patent application No US20090190232 of the same applicant, the content of which is incorporated herein by reference. This document discloses a lens barrel comprising a liquid lens and a number of fixed lenses, where said lens barrel has contact regions to align the fixed lenses using their outer diameter. It addresses the problem of centring the fixed lenses with respect to the optical axis of a lens stack. Patent application No EP1992968 of the same applicant, the content of which is incorporated herein by reference, discloses vertical positioning of fixed lenses in a lens barrel comprising a liquid lens, wherein the fixed lenses seal the liquid lens. Patent application No EP1884805 of the same applicant, the content of which is incorporated herein by reference, discloses passive alignment means for a liquid lens having a two-part body and two fixed lenses. Patent application JP 2007171329 discloses a lens barrel comprising a liquid lens and alignment mechanisms therefor.
However, the entire mechanical datum of the lens to be aligned (by centring) with respect to other lenses is not always available, or the optical axis of the lens to be aligned is not well aligned with respect to its mechanical datum (i.e., its outer diameter OD) during the manufacturing process. It is therefore desirable to provide location features that could avoid active alignment.
Another constraint is that electrical contact with the liquid lens needs to be provided to the two electrodes (for example electrodes being respectively the bodycone and the cap of the liquid lens). Flexible Printed Circuit (FPC) gluing could be a solution, however this solution requires a double tape conductive glue/tape that are known to be not reliable and to create contact failure during the assembly process. Other solutions such as spring metal solutions described in EP1992968 could be used and are more reliable than a FPC gluing, however the pressure applied through the metal spring contact can modify the position of the liquid lens, and thus deteriorate the optical alignment of the liquid lens and the optical elements.
Most of the low profile lens module for mobile phone application are so called front aperture system. The front aperture system allows to minimise the chief ray angle (CA) on the sensor. Low CA are required to have efficient micro lenses array which will benefit to the overall sensitivity of the imaging system. In the particular when using the liquid lens in such a module, the aperture stop is placed on the front lens on the liquid lens.
Another important aspect relates to the control of the distance between the lenses in the lens module with a high accuracy. Accuracy on the position along the optical axis between each lens of a lens system are typically ranging from 5-10 microns. The most critical distance is usually the distance between the lenses that are on the opposite side of the sensor. In particular when using a liquid lens comprising at least one fixed lens (instead of two windows being transparent plates), one critical parameter is the distance between the fixed lens of the liquid lens and the next plastic or glass lens of the lens arrangement of the lens assembly. A datum B of the liquid lens could be used as a mechanical reference, but the accuracy is very easily ranging around 15-30 microns for the position of the lens with respect to the datum B. Such level of accuracy is suitable to insure high level of MTF (over 70% MTF on axis and over 40% MTF at 80% FOV @ ⅓ of the Niquist Frequency related to the sensor resolution) for low profile lens module.
Experience has shown that to get very accurate Glass moulded with Datum B below 15 μm, the production yield is ranging around 80% with a very poor reliability performances.
The invention hereafter described is to overcome at least one of the above problems related to the use of a liquid lens integrating a fixed lens, in a camera module.